Sail with New Freebooters

In the previous post of this series, we calculated the typical worst case demand on our boat’s power systems during a 12-hour period. Now let’s decide what we need in the way of batteries to supply this demand.

First, let’s look at the different types of battery on the market in 2009, and decide which are suitable for marine use.

LEAD-ACID BATTERIES

Each cell in a fully-charged lead-acid battery consists of a lead oxide (PbO2) positive plate and a lead negative plate, immersed in dilute sulphuric acid ‘electrolyte’. As it discharges, the surfaces of both plates gradually turn to lead sulphate, and recharging reverses this process. During the last phase of recharging, some of the water is given off as hydrogen and oxygen gas bubbles. This, known as a ‘flooded cell’ or ‘wet’ battery, is the toughest and cheapest kind , but it has disadvantages:

• The cells need to be topped up with distilled water from time to time. This isn’t a great problem unless you are doing a very long ocean passage, but failure to do so will damage the battery.
• The box in the boat which houses the batteries needs to be designed so that it contains any acid which spills when the boat heels too far over. Some people claim that battery acid reacts with salt water to produce chlorine, an extremely poisonous gas – No battery of this type can tolerate the boat being laid flat or being completely inverted.
• The mix of hydrogen and oxygen given off at the end of the charge cycle is highly inflammable and potentially explosive. Battery compartments must be well ventilated, and should be well away from any source of  sparks or flames.

To overcome the above problems, the first Sealed Lead-Acid (SLA) batteries appeared in the 1950s, originally intended for use in aircraft. Because the cells run pressurised and incorporate a safety vent, Sealed Lead-Acid batteries were also know as Valve Regulated Lead Acid (VRLA) batteries. Since then the technology has developed to the point where they are an excellent choice for marine use.

The main characteristics of all SLA or VRLA cells are:

• The electrolyte is immobilised so that it can’t spill, by using either a gel or a very fine glass mat.
• The cell is sealed and kept at about 10% above atmospheric pressure to encourage re-absorption of the gas given off in the last phase of charging. A safety vent allows gas to escape if pressure builds up too much (which will happen if the cell is over-charged).
• The plate material is slightly modified to stop hydrogen being given off at all unless the cell is overcharged. Under normal charging conditions, oxygen passes through the mat or through microfissures in the gel without excess pressure buildup.

The first Sealed Lead-Acid batteries used a fine silica gel to hold the electrolyte, and enhanced versions of this Gel design are still available. In the 1970s, Absorbed Glass Mat became the leading SLA technology. This uses a mat of very fine glass fibres to retain the electrolyte. Both can be turned completely upside-down without leaking, and AGM batteries can accept a very high charging current – so they can be recharged more quickly than wet cell batteries.

Both Gel and AGM cells lose contact between the electrolyte and the plates with age. Glass mat has to press very firmly against the plates, and loses its springiness, while gel shrinks as it dries out.

Gel and AGM batteries need different charging regulator settings than wet cell batteries – they are not drop-in replacements for wet cell batteries, and they must not be mixed with wet cell batteries in one power pack, or the resulting overcharging will destroy them.

Battery developers are, of course, continually working to solve these problems, and as I write in January 2009 AGM is the favorite for standby applications such as uninterruptible power supply (UPS) batteries, but is also used for recreational boating. Gel cell batteries are claimed to be the best for cyclical rather than standby use, but very few are sold compared with the number of AGM ones.

Some battery suppliers also argue that marine applications are not purely cyclic – in real life, the daily cycle is interspersed with long periods where the boat sits unused in the marina. They suggest that a new technology – the Hybrid Gel cell – is the best solution. This uses the tradtitonal AGM structure, but surrounds it with an improved gel – a polysilica colloid. The manufacturers claim the following advantages over traditional Gel or AGM batteries:

• Superior cyclic service, even in adverse environmental conditions.
• Improved thermal properties, extending long-term reliability and performance.

None of the alternatives to Lead-Acid batteries has yet been developed to provide a reasonable alternative fro marine house power use, so the choice is between the cheaper and more electrically tolerant wet cell battery which doesn’t tolerate extreme angles of heel, and the more expensive and sensitive AGM battery which works well in extreme weather and temperature conditions. When used for standby poser systems, where thye spend most of their life doing nothing but have their self-discharge losses replenished, AGM batteries have a much shorter life than suitably-designed wet cell batteries, but in typical marine use the difference is not so great – in fact, wet cell batteris that are not rigorously kept charged and topped up (a situation quite common in recreational boats) could fail first.

OTHER BATTERY TYPES

Let’s dismiss them quickly:

- NiCad batteries are useful if you need them to work at -40 degrees (same in both C and F scales), but very expensive to buy and to dispose of

- NiFe batteries don’t give a steady voltage as they discharge. Very few are made nowadays

- Fuel Cells are still tomorrow’s technology.

That’s enough forthis session. The next post will deal with how to charge the batteries on your boat.